The present invention relates generally to optical modulators and associated modulation methods and, more particularly, to an optical modulator utilizing an optical fiber as the active medium and an associated modulation method.
An increasing number of telecommunications, computer and other networks rely upon optical fibers, as opposed to electrical wiring, for signal transmission. Each of these optical networks typically includes a number of optical modulators for encoding the data to be transmitted via the optical fibers so that the encoded data can ride upon an optical carrier signal. As such, optical modulators are in great demand with increasing emphasis being placed upon the speed at which the optical modulator operates and the cost of the optical modulator.
While optical modulators are widely utilized in conventional optical networks, the demand for optical modulators will likely grow as internet service providers offer an increasing number of wideband services to both businesses and residential customers. Unfortunately, the conventional optical modulators that are capable of modulating signals over a wide frequency band are disadvantageously expensive, as described in more detail below. In fact, optical modulators may be the most costly components of some fiber optic communication systems.
A conventional optical modulator includes a single crystal substrate formed of a ferroelectric material that lacks internal symmetry, such as single crystal lithium niobate (LiNbO3). A ferroelectric material possesses a spontaneous dielectric polarization and, consequently, also exhibits a linear electrooptic effect, i.e., a Pockels effect. As a result, the refractive index of a single crystal substrate exhibiting a linear electrooptic effect changes linearly with an applied electric field. Accordingly, optical signals propagating through an in-diffused single mode waveguide fabricated on a single crystal substrate exhibiting a linear electrooptic effect can be phase modulated by applying an appropriate electric field. Typically, the electric field is created by applying an electrical signal to electrodes on opposite sides of the single crystal substrate.
An optical modulator, such as an optical modulator fabricated from single crystal lithium niobate, can be characterized by its half-wave switching voltage VS and its electrical bandwidth xcex94f. The half-wave switching voltage is the voltage that must be applied across the single crystal substrate in order to induce a phase shift of xcfx80. It is generally desirable for the half-wave switching voltage to be less than about 10 volts such that solid state driver amplifiers can be utilized to create the electric field across the single crystal substrate. In addition, the electrical bandwidth of an optical modulator is the frequency band over which the modulation response remains within 3 dB of the peak value. For example, the electrical bandwidth of a conventional optical modulator is in the tens of gigahertz.
In order to fabricate an optical modulator, the single crystal material is drawn from a high temperature melt according to a Czochralski process. The crystal is then poled by annealing the crystal at elevated temperatures in the presence of an applied electrical field that aligns the ferroelectric domains in the same direction. The crystal is next cut into thin wafers and the major surfaces of each wafer are polished to an optical grade finish. Utilizing photolithography, a mask that defines the desired waveguide pattern is then formed upon a polished surface of the substrate. After processing the photoresist, titanium metal is typically deposited on the surface and is then in-diffused at high temperatures to form the single mode waveguides. Metal electrodes are then deposited on the polished surface of the substrate in the same pattern of the waveguides. Since a plurality of modulators can be formed upon a single substrate, the individual optical modulators are then cut or otherwise separated from the remainder of the wafer. The optical modulator is then packaged with fiber optic pigtails and radio frequency (RF) connections in a hermetic package with corresponding single mode optical fiber connectors.
Although the resulting optical modulator can reliably phase modulate the optical signals transmitted via the single mode waveguides within an optical network, the process for fabricating a conventional optical modulator requires precise dimensional control and is relatively expensive. As such, the resulting optical modulators also are disadvantageously expensive, especially relative to other components within an optical network. As such, it would be desirable to provide reliable optical modulators that can be fabricated in a less expensive manner.
An optical modulator and an associated modulation method are therefore provided that utilize an optical fiber as the active medium such that the resulting fiber modulator is less expensive, but maintains high performance standards. In this regard, the fiber modulator of the present invention includes a core having a first index of refraction and a cladding surrounding the core and having a second index of refraction that is less than the first index of refraction. As such, the core and the surrounding cladding generally form a longitudinally extending optical fiber capable of supporting the propagation of optical signals through the core thereof. The fiber modulator also includes first and second regions within the cladding and extending longitudinally therealong for establishing an internal bias electrical field across the core. The first and second regions are therefore disposed on opposite sides of the core and have positive and negative electrical charges, respectively. The fiber modulator further includes first and second electrodes disposed on the cladding proximate the first and second regions, respectively, and extending longitudinally therealong. By applying electrical signals, such as radio frequency (RF) signals, to the first and second electrodes, the optical signals propagating through the core of the optical fiber can be linearly phase modulated.
In one advantageous embodiment, the optical fiber and, more particularly, the cladding has a rectangular shape in lateral cross section. As such, the optical fiber has a pair of opposed major surfaces and a pair of opposed minor surfaces. In this embodiment, the first and second regions and the first and second electrodes are all preferably disposed proximate a respective major surface of the optical fiber. For example, the first region and the first electrode can be disposed proximate a first major surface and the second region and the second electrode can be disposed proximate an opposed second major surface.
Preferably, the optical fiber is a single mode fiber such that the core is adapted to support optical signal propagation in a single mode. In addition, the optical fiber may serve not only as the active medium of a fiber modulator, but also as an amplifier, i.e., a fiber amplifier. In this instance, the core may be doped with a rare earth dopant to thereby amplify the optical signals propagating therethrough if the optical fiber is also appropriately pumped.
In operation, an internal DC bias electrical field is established across the core. According to the present invention, the internal DC bias electrical field is established by the first and second regions that have positive and negative electrical charges, respectively, and that are positioned on opposite sides of the core. While the internal DC bias electrical field is applied across the core, electrical signals are also applied to the electrodes that extend lengthwise along opposite sides of the optical fiber to thereby linearly phase modulate the optical signals propagating through the core of the optical fiber. Typically, the electrical signals applied to the electrodes are RF signals. In one embodiment, for example, the RF signals can be applied to one end of the electrodes such that the RF signals propagate lengthwise along the electrodes concurrent with the propagation of the optical signals through the core of the optical fiber. Preferably, the electrical signals that are applied to the electrodes are selected such that the resulting electrical field established by the electrical signals is smaller than the internal DC bias electrical field. As such, the internal DC bias electrical field can induce a linear electrooptic effect within an optical fiber that would otherwise exhibit a quadratic electrooptic effect, i.e., a Kerr effect. As a result of the linear electrooptic effect, the smaller electrical field created by the electrical signals applied to the electrodes serves to linearly modulate the optical signals propagating through the optical fiber. Concurrent with the modulation of the optical signals, the optical signals can be amplified if the core of the optical fiber has been appropriately doped with a rare earth dopant and is optically pumped.
Since it is premised upon optical fiber technology, the fiber modulator of the present invention can therefore be fabricated at much lower cost than conventional optical modulators. As such, the resulting fiber modulator generally is less expensive. However, a fiber modulator of the present invention that includes an optical fiber as the active medium still maintains high performance levels with respect to the linear phase modulation of the optical signals propagating therealong.